Controller IC, DC-AC conversion apparatus, and parallel running system of DC-AC conversion apparatuses

ABSTRACT

A DC power source voltage is supplied to a center tap of a primary winding, and first and second semiconductor switches alternately turned on are disposed between each of both ends of the primary winding and a common potential point, and a current flowing through a load is fed back and PWM control of each of the semiconductor switches is performed. Also, snubber circuits are respectively connected between a ground and the center tap of the primary winding, and an abnormal high voltage at the time of switching is reduced. Also, a parallel running of plural inverters is simply performed by disposing PWM comparators corresponding to the first and second semiconductor switches.

This is a continuation of application Ser. No. 11/666,492, filed on Apr.27, 2007 now U.S. Pat. No. 7,394,641 and claims the benefit of JapanesePatent Application No. 2004-329572, filed in Japan on Nov. 12, 2004,which are hereby incorporated by reference.

The present invention relates to a DC-AC conversion apparatus(hereinafter called an inverter) for generating an AC voltage fordriving a load from a DC power source of a battery etc. or a powersource adapter attached to electrical equipment, a controller IC used inthe inverter, and a parallel running system of plural inverters.

BACKGROUND OF THE INVENTION Field of the Invention

A cold cathode fluorescent lamp (CCFL) has been used as a backlightlight source of a liquid crystal display such as a liquid crystaltelevision set or a liquid crystal monitor of a notebook personalcomputer. This CCFL generally has higher efficiency and longer life thanthose of a normal hot cathode fluorescent lamp and then, a filamentwhich the hot cathode fluorescent lamp has is omitted.

A high AC voltage is required in order to start and operate this CCFL.For example, a starting voltage is about 1000 V and an operating voltageis about 600 V. This high AC voltage is generated from a DC power sourceof a notebook personal computer, a liquid crystal television set, etc.using an inverter.

Conventionally, a Royer circuit has generally been used as an inverterfor CCFL. This Royer circuit is constructed of a saturable magnetic coretransformer, a control transistor, etc., and then self-oscillates bycharacteristics of non-linear current gain of the control transistor andnon-linear magnetic permeability of the saturable magnetic coretransformer. The Royer circuit itself does not require an external clockor a driver circuit.

However, the Royer circuit is basically a constant-voltage inverter, andcannot maintain a constant output voltage when an input voltage or aload current varies. Therefore, a regulator for supplying electric powerto the Royer circuit is required. Because of this, it is difficult tominiaturize the inverter using the Royer circuit and also, electricpower conversion efficiency is low.

An inverter using a center tap type transformer having a primary windingin which a DC voltage is supplied to a center tap and a secondarywinding for AC voltage output has been proposed (see JP-T-2002-500427and JP-A-6-14556).

In the inverter of JP-T-2002-500427, a DC voltage is supplied to acenter tap of a primary winding and semiconductor switches arerespectively had between a ground and each end of the primary windingand their semiconductor switches are alternately turned on and off. APWM control apparatus for performing PWM control of a DC voltagesupplied to its inverter is disposed. Then, electric power supplied fromthe inverter to a load is controlled by control of a DC current by thePWM control apparatus.

The inverter of JP-A-6-14556 comprises a step-up transformer having aprimary winding in which a DC power source is connected to a center tap,a secondary winding for AC voltage output and a tertiary winding forfeedback, a resonance capacitor which is connected between both ends ofthe primary winding of its step-up transformer and constructs an LCresonance circuit between the resonance capacitor and an inductance ofthis primary winding, a pair of semiconductor switches alternatelyturned on and off by an output voltage of the tertiary winding, one endsides of the semiconductor switches being respectively connected to thedifferent ends of its resonance capacitor and the other end sides beingearthed, and a variable inductor connected to the inside of its LCresonance circuit. Then, an output voltage of the inverter is controlledby controlling an inductance of the variable inductor.

Means using a Royer circuit has problems that it is difficult tominiaturize the means and also conversion efficiency is low. In theinverter of JP-T-2002-500427, the PWM control apparatus for performingPWM control of a DC voltage supplied to its inverter is required inaddition to the inverter, so that a structure as the whole DC-ACconversion apparatus becomes complicated and also it is difficult tominiaturize the inverter. Also, the inverter of JP-A-6-14556 comprisesthe variable inductor connected to the inside of the LC resonancecircuit and an output voltage is controlled by controlling itsinductance, so that a structure becomes complicated and also it isdifficult to miniaturize the inverter.

Further, with an increase in screen size of a liquid crystal displaysuch as a liquid crystal television set or a liquid crystal monitor of anotebook personal computer, plural CCFLs have been distributed andarranged as a backlight light source. In this case, light from theplural CCFLs mutually interferes and becomes a cause of flicker etc., sothat it becomes necessary to synchronously turn on each of the CCFLs inthe same phase.

For this purpose, it is contemplated to construct an inverter of adiscrete circuit and supply AC power of the same phase to plural CCFLs.

However, by reasons that it is necessary to reduce an influence on otherapparatus by decreasing a routed distance of high-voltage wiring to theCCFL and also that parasitic capacitance of the CCFL is effectively usedin resonance with a transformer, it is desirable that the inverter forcontrolling each of the CCFLs be arranged as close as possible to theCCFLs.

JP-T-2002-500427 and JP-A-6-14556 are seen as the related art.

SUMMARY OF THE INVENTION

An object of the invention is to provide a DC-AC conversion apparatus(an inverter) for generating an AC voltage for driving a load from a DCpower source, a DC-AC conversion apparatus suitable for parallel runningof plural loads, the DC-AC conversion apparatus capable of finelyadjusting electric power supply to the loads by a simple configurationusing a transformer having a primary winding with a center tap to whicha DC voltage is supplied, a controller IC used in the DC-AC conversionapparatus, and a parallel running system for synchronously controllingplural DC-AC conversion apparatus in the same phase.

The invention provides a controller IC for controlling a firstsemiconductor switch and a second semiconductor switch for driving aload, having an oscillator block for generating a first triangular wavesignal and a second triangular wave signal having a relation in whichthe first triangular wave signal is reversed when a capacitor forfrequency decision and a resistor for frequency decision are connected,a pulse width modulation circuit for comparing the first triangular wavesignal with a feedback signal formed based on a current flowing throughthe load and generating a first pulse width modulation signal andcomparing the second triangular wave signal with the feedback signal andgenerating a second pulse width modulation signal, and a driving signaloutput block for alternately outputting a first switch driving signalfor turning on the first semiconductor switch based on the first pulsewidth modulation signal and a second switch driving signal for turningon the second semiconductor switch based on the second pulse widthmodulation signal, characterized in that the driving signal output blockgenerates the first switch driving signal and the second switch drivingsignal at timing at which an off period during which both of the firstsemiconductor switch and the second semiconductor switch are turned offis set between a period for which the first semiconductor switch is inthe on state and a period for which the second semiconductor switch isin the on state.

In the controller IC, the oscillator block to which the capacitor forfrequency decision and the resistor for frequency decision are notconnected generates a triangular wave signal supplied from the outsideand a triangular wave signal having a relation in which the triangularwave signal is reversed.

In the controller IC, the driving signal output block outputs the firstswitch driving signal from a point in time of one vertex of the firsttriangular wave signal to a point in time when the first triangular wavesignal immediately after the point in time becomes equal to the feedbacksignal, and outputs the second switch driving signal from a point intime of the one vertex of the second triangular wave signal to a pointin time when the second triangular wave signal immediately after thepoint in time becomes equal to the feedback signal.

The invention also provides a DC-AC conversion apparatus having atransformer having a primary winding with a center tap and at least onesecondary winding, the center tap being connected to a first potentialpoint of a DC power source, a first semiconductor switch connectedbetween one end of the primary winding and a second potential point ofthe DC power source, a second semiconductor switch connected between theother end of the primary winding and the second potential point, acurrent detection circuit for detecting a current flowing through a loadconnected to the secondary winding, an oscillation circuit forgenerating a first triangular wave signal and a second triangular wavesignal having a relation in which the first triangular wave signal isreversed, a pulse width modulation circuit for comparing the firsttriangular wave signal with a feedback signal formed based on a currentdetected by the current detection circuit and generating a first pulsewidth modulation signal and comparing the second triangular wave signalwith the feedback signal and generating a second pulse width modulationsignal, and a driving signal output circuit for alternately outputting afirst switch driving signal for turning on the first semiconductorswitch based on the first pulse width modulation signal and a secondswitch driving signal for turning on the second semiconductor switchbased on the second pulse width modulation signal, characterized in thatthe driving signal output circuit generates the first switch drivingsignal and the second switch driving signal at timing at which an offperiod during which both of the first semiconductor switch and thesecond semiconductor switch are turned off is set between a period forwhich the first semiconductor switch is in the on state and a period forwhich the second semiconductor switch is in the on state.

In the DC-AC conversion apparatus, the driving signal output circuitoutputs the first switch driving signal from a point in time of onevertex of the first triangular wave signal to a point in time when thefirst triangular wave signal immediately after the point in time becomesequal to the feedback signal, and outputs the second switch drivingsignal from a point in time of the one vertex of the second triangularwave signal to a point in time when the second triangular wave signalimmediately after the point in time becomes equal to the feedbacksignal.

The DC-AC conversion apparatus has a first snubber circuit connectedbetween one end of the primary winding and the second potential pointand a second snubber circuit connected between the other end of theprimary winding and the second potential point.

The DC-AC conversion apparatus has a first snubber circuit connectedbetween one end of the primary winding and the first potential point anda second snubber circuit connected between the other end of the primarywinding and the first potential point.

In the DC-AC conversion apparatus, the first potential point is acontact point to which a power source voltage of the DC power source isapplied and the second potential point is a ground.

The invention also provides a parallel running system of DC-ACconversion apparatuses, having a transformer having a primary windingwith a center tap and at least one secondary winding, the center tapbeing connected to a first potential point of a DC power source, a firstsemiconductor switch connected between one end of the primary windingand a second potential point of the DC power source, a secondsemiconductor switch connected between the other end of the primarywinding and the second potential point, a current detection circuit fordetecting a current flowing through a load FL connected to the secondarywinding, an oscillation circuit for generating a first triangular wavesignal and a second triangular wave signal having a relation in whichthe first triangular wave signal is reversed when a capacitor forfrequency decision and a resistor for frequency decision are connectedand also generating a triangular wave signal supplied from the outsideand a triangular wave signal having a relation in which the triangularwave signal is reversed when the capacitor for frequency decision andthe resistor for frequency decision are not connected, a pulse widthmodulation circuit for comparing the first triangular wave signal with afeedback signal formed based on a current detected by the currentdetection circuit and generating a first pulse width modulation signaland comparing the second triangular wave signal with the feedback signaland generating a second pulse width modulation signal, and a drivingsignal output circuit for alternately outputting a first switch drivingsignal for turning on the first semiconductor switch based on the firstpulse width modulation signal and a second switch driving signal forturning on the second semiconductor switch based on the second pulsewidth modulation signal, the parallel running system having plural DC-ACconversion apparatuses in which the driving signal output circuitgenerates the first switch driving signal and the second switch drivingsignal at timing at which an off period during which both of the firstsemiconductor switch and the second semiconductor switch are turned offis set between a period for which the first semiconductor switch is inthe on state and a period for which the second semiconductor switch isin the on state, characterized in that the capacitor for frequencydecision and the resistor for frequency decision are connected to theoscillation circuit in only one of the plural DC-AC conversionapparatuses, and the first DC-AC conversion apparatus having theoscillation circuit to which the capacitor for frequency decision andthe resistor for frequency decision are connected supplies only thefirst triangular wave signal of the first triangular wave signal and thesecond triangular wave signal generated from the oscillation circuit tothe DC-AC conversion apparatuses other than the first DC-AC conversionapparatus, and the plural DC-AC conversion apparatuses perform pulsewidth modulation control of the same phase synchronously respectivelyusing the first triangular wave signal and the second triangular wavesignal.

In the parallel running system of the DC-AC conversion apparatuses, aresistance value of the resistor for frequency decision is set atdifferent values after starting and at the time of starting of the DC-ACconversion apparatus, and the resistance value set at the time of thestarting is smaller than the resistance value set after the starting.

According to the controller IC, the DC-AC conversion apparatus and theparallel running system of the DC-AC conversion apparatuses describedabove, in the DC-AC conversion apparatus for generating an AC voltagefor driving a load from a DC power source, a transformer having aprimary winding with a center tap to which a DC power source voltage issupplied is used and a first semiconductor switch and a secondsemiconductor switch alternately turned on are disposed between each endof the primary winding and a common potential point and a currentflowing through a load is fed back and pulse width modulation (PWM)control of each of the semiconductor switches is performed, so thatelectric power supply to the load can be finely adjusted by a simpleconfiguration.

Also, a pulse width modulation circuit generates a first pulse widthmodulation signal for controlling a first semiconductor switch bycomparison between a first triangular wave signal and a feedback signal,and generates a second pulse width modulation signal for controlling asecond semiconductor switch by comparison between a feedback signal anda second triangular wave signal having a relation in which the firsttriangular wave signal is reversed. Therefore, in the case of parallelrunning plural DC-AC conversion apparatuses, only the first triangularwave signal is simply transmitted between the DC-AC conversionapparatuses, so that synchronization between the plural DC-AC conversionapparatuses can be obtained more simply than ever before.

Also, snubber circuits are respectively disposed between a low impedancepoint (that is, a second potential point or a first potential point) andboth ends of a primary winding. As a result of this, occurrence of anabnormal high voltage applied to a first semiconductor switch and asecond semiconductor switch at the time of switching can be suppressed.Particularly, in a push-pull configuration, a voltage two times thepower source voltage is always applied, so that suppression of theabnormal voltage is particularly effective.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the whole configuration of an inverteraccording to an embodiment of the invention;

FIG. 2 is a diagram showing an internal configuration of an IC 200 forinverter control for FIG. 1;

FIG. 3 is a diagram showing a circuit configuration example of a PWMblock of FIG. 2;

FIG. 4 is a timing chart of the inverter of FIG. 1;

FIG. 5 is a diagram showing a configuration of a parallel running systemof the inverters of the invention;

FIG. 6 is a diagram showing an operating state at each timing of FIG. 4;

FIG. 7 is a voltage waveform diagram of a primary winding of atransformer in the inverter of the invention;

FIG. 8 is a voltage waveform diagram of a primary winding of atransformer in an inverter used as the related art; and

FIG. 9 is a diagram showing another configuration example of a snubbercircuit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Description ofReference Numerals and Signs

TR TRANSFORMER WITH CENTER TAP

-   -   FL COLD CATHODE FLUORESCENT LAMP    -   BAT DC POWER SOURCE (BATTERY POWER SOURCE)    -   101,102 FIRST, SECOND SEMICONDUCTOR SWITCH    -   103,105 RESISTOR FOR SNUBBER    -   104,106 CAPACITOR FOR SNUBBER    -   200,200A˜200N IC FOR INVERTER CONTROL    -   201 OSC BLOCK    -   202 BOSC BLOCK    -   203 LOGIC BLOCK    -   204 OUTPUT BLOCK    -   250 PWM BLOCK    -   251,252 FIRST, SECOND PWM COMPARATOR    -   VDD POWER SOURCE VOLTAGE    -   VCC POWER SOURCE VOLTAGE    -   N1,N2 GATE DRIVING SIGNAL    -   STB STARTING SIGNAL    -   CT FIRST TRIANGULAR WAVE SIGNAL FOR PWM    -   CTI SECOND TRIANGULAR WAVE SIGNAL FOR PWM    -   FB FEEDBACK VOLTAGE    -   IS DETECTION CURRENT    -   SS SLOW START VOLTAGE    -   BCT TRIANGULAR WAVE SIGNAL FOR BURST    -   DUTY DUTY SIGNAL FOR BURST

An embodiment of an inverter for generating an AC voltage for driving aload from a DC power source and a parallel running system of theinverters of the invention will be described below with reference to thedrawings.

FIG. 1 is a diagram showing the whole configuration of an inverteraccording to an embodiment of the invention for performing PWM controlusing a semiconductor switch circuit and an insulating transformer TRhaving a primary winding with a center tap and a secondary winding. FIG.2 is a diagram showing an internal configuration of an IC 200 forinverter control therefor.

In FIG. 1, the transformer TR is an insulating transformer having asecondary winding 108 for supplying AC electric power to a load, and aprimary winding 107 having a center tap T, a terminal (hereinaftercalled a first terminal) A of one end and a terminal (hereinafter calleda second terminal) B of the other end. A power source voltage VDD from abattery power source BAT is supplied to the center tap T of thistransformer TR. This DC power source voltage VDD is a voltage withrespect to a ground GND which is a common potential point.

Also, the voltage VDD of the battery power source BAT is adjusted to apredetermined power source voltage VCC (for example, 5 to 14 V) of theIC 200 by a series regulator made of a resistor 121, a Zener diode 122and an NPN type bipolar transistor (hereinafter called an NPN) 123, andis supplied to the IC 200. In addition, a capacitor 124 is a capacitorfor stability.

An N type MOSFET (hereinafter called an NMOS) 101 which is a firstsemiconductor switch is a switch for forming a current path of a firstdirection to the primary winding 107 of the transformer TR. Also, anNMOS 102 which is a second semiconductor switch is a switch for forminga current path of a second direction to the primary winding 107 of thetransformer TR. An alternating current flows through the primary winding107 of the transformer TR by alternately turning on and off this NMOS101 and the NMOS 102.

Also, a first snubber circuit made of a series circuit of a firstresistor 103 and a first capacitor 104 is connected between the groundand the first terminal A of the primary winding 107. Similarly, asnubber circuit made of a series circuit of a second resistor 105 and asecond capacitor 106 is connected between the ground and the secondterminal B of the primary winding 107. These first and second snubbercircuits suppress an abnormal high voltage such as a surge voltageoccurring in the first and second terminals A, B.

These NMOS 101 and NMOS 102 have body diodes (that is, back gatediodes), respectively. A current of a direction opposite to the originalcurrent path can be passed by this body diode. In addition, a diode forperforming a function similar to the body diode may be disposedseparately.

The power source voltage VDD of the DC power source BAT is supplied tothe primary winding 107 of the transformer TR through the NMOS 101 andthe NMOS 102, and a high voltage according to a winding ratio is inducedin its secondary winding 108. This induced high voltage is supplied to acold cathode fluorescent lamp FL which is a load, and the cold cathodefluorescent lamp FL lights up.

Together with a capacitor 111, a capacitor 112, a resistor 117 and aresistor 118, a voltage applied to the cold cathode fluorescent lamp FLis detected and is fed back to the controller IC 200. A resistor 114 anda resistor 115 detect a current flowing through the cold cathodefluorescent lamp FL and feed back the current to the controller IC 200.Also, the capacitor 111 is means for producing resonance by itscapacitance and an inductance component of the transformer TR, andparasitic capacitance of the cold cathode fluorescent lamp FL alsocontributes to this resonance. Numerals 113, 116, 119, 120 are diodes.

The controller IC 200 has plural input-output pins. A first pin 1P is aterminal of switching between a PWM mode and an intermittent operation(hereinafter called a burst) mode, and a duty signal DUTY for deciding aduty ratio at the time of the burst mode and switching of their modes isinputted from the outside. A second pin 2P is an oscillation frequencysetting capacity connection terminal of a burst mode oscillator (BOSC),and a capacitor 131 for setting is connected and a triangular wavesignal BCT for burst is generated. A third pin 3P is an oscillationfrequency setting resistance connection terminal of the BOSC, and aresistor 132 for setting is connected.

A fourth pin 4P is an oscillation frequency setting capacity connectionterminal of a PWM mode oscillator (OSC), and a capacitor 133 for settingis connected and a triangular wave signal CT for PWM is generated. Afifth pin 5P is a resistance connection terminal for setting a chargingcurrent to the fourth pin 4P, and a resistor 134 for setting isconnected and a current according to its potential RT and a resistancevalue flows.

A sixth pin 6P is a resistance connection terminal for setting acharging current to the fourth pin 4P, and a resistor 135 for setting isconnected and by control of an internal circuit, this resistor 135 isconnected in parallel with the resistor 134 for setting or disconnectedand its potential SRT becomes a ground potential GND or the potential RTof the fifth pin 5P. A seventh pin 7P is a ground terminal and is in theground potential GND.

An eighth pin 8P is a first error amplifier output terminal and acapacitor 136 is connected between this eighth pin 8P and a ninth pin9P. A potential of the eighth pin 8P becomes a feedback voltage FB andbecomes a control voltage for PWM control. Hereinafter, each of thevoltages is based on the ground potential unless otherwise specified.

A current detection signal (hereinafter called a detection current) ISaccording to a current flowing through the cold cathode fluorescent lampFL is inputted to the ninth pin 9P through a resistor 138 and isinputted to a first error amplifier.

A voltage detection signal (hereinafter called a detection voltage) VSaccording to a voltage applied to the cold cathode fluorescent lamp FLis inputted to a tenth pin 10P through a resistor 139 and is inputted toa second error amplifier. A capacitor 137 is connected between the tenthpin 10P and the eighth pin 8P.

An eleventh pin 11P is a starting signal input terminal, and a startingsignal STB is applied. A twelfth pin 12P is a blank terminal in thisexample.

A thirteenth pin 13P is a terminal for regulator and is connected to aregulator block in its inside. A capacitor 142 connected to the pin 13Pis used for stability operation of the regulator.

A fourteenth pin 14P is a slow start setting capacity connectionterminal and a capacitor 141 is connected between the pin 14P and theground, and a voltage SS for slow start for gradually increasing at thetime of starting is generated.

A fifteenth pin 15P is a timer latch setting capacity connectionterminal, and a capacitor 140 for deciding operation timing for internalprotection operation is connected, and a potential SCP according to anelectric charge of the capacitor 140 is generated.

A sixteenth pin 16P is a terminal for outputting a gate driving signalN2 of the NMOS 102. A seventeenth pin 17P is a ground terminal PGND ofan external FET drive circuit. An eighteenth pin 18P is a terminal foroutputting a gate driving signal N1 of the NMOS 101.

A nineteenth pin 19P is a terminal FAIL for outputting a failuredetection signal detected in the inside to the outside. A twentieth pin20P is a power source terminal for inputting the power source voltageVCC in the IC 200.

In FIG. 2 showing the internal configuration of the controller IC 200,an OSC block 201 generates a first triangular wave signal CT for PWM inwhich a cycle etc. are decided by the resistors 134, 135 connected tothe fifth pin 5P and the capacitor 133 connected to the fourth pin 4Pand a second triangular wave signal CTI having a relation in which itsfirst triangular wave signal is reversed, and supplies the triangularwave signals to a PWM block 250 including a PWM comparator, and alsogenerates an internal clock CLK in synchronization with the firsttriangular wave signal CT, and supplies the internal clock to a logicblock 203. An oscillation circuit for PWM is constructed by this OSCblock 201, the capacitor 133 and the resistors 134, 135.

This first triangular wave signal CT and the second triangular wavesignal CTI having the relation reverse to its first triangular wavesignal are preferably a triangle (that is, an isosceles triangle) inwhich time of a rise interval is equal to time of a fall interval.

Also, the OSC block 201 does not generate the triangular wave signalsCT, CTI when the resistor 134 etc. are not connected to the fifth pin5P. However, when, for example, the first triangular wave signal CT isinputted from the outside to the fifth pin 5P, the second triangularwave signal CTI having a relation reverse to its first triangular wavesignal is formed. Also, the clock CLK is formed. Therefore, also in thiscase, the first and second triangular wave signals CT, CTI and the clockCLK can be used in the inside of the IC 200.

A BOSC block 202 generates the triangular wave signal BCT for burst of acycle decided by the capacitor 131 connected to the second pin 2P or theresistor 132 etc. connected to the third pin 3P. A frequency (BCTfrequency) of the triangular wave signal BCT for burst is set remarkablylower than a frequency of the first triangular wave signal CT (BCTfrequency<CT frequency).

An analog (DC voltage) duty signal DUTY supplied to the first pin 1P iscompared with the triangular wave signal BCT for burst by a comparator221 and an NPN 234 is driven by this comparison output through an ORcircuit 239. In addition, when a digital (PWM format) duty signal DUTYis supplied to the first pin 1P, a resistor is connected to the secondpin 2P and a predetermined voltage for burst is generated from the BOSCblock 202.

In the logic block 203, first and second PWM control signals, a clocksignal, etc. are inputted and first and second switch control signalsare generated according to predetermined logic. Also, an output block204 forms first and second switch driving signals (gate driving signals)N1, N2 according to the first and second switch control signals, andapplies the driving signals to gates of the NMOSs 101, 102.

When a starting signal STB is inputted, an input to a comparator 217exceeds its reference voltage Vref6, so that an off state of a system isreleased by an output of the comparator 217 and the inside of the IC 200becomes an operating state. Also, the output of the comparator 217 issupplied to the logic block 203 through a reverse circuit 249 and candrive the logic block 203.

At the same time, a slow start block 205 is started by the output of thecomparator 217. When the slow start block 205 is started, an externalcapacitor 141 is charged and also a slow start voltage SS increasesgradually. This slow start voltage SS is supplied to the PWM block 250and is used in a slow start of PWM control. Therefore, the PWM controlis performed according to the slow start voltage SS at the time ofstarting.

Also, by the output of the comparator 217, a flip-flop (FF) circuit 242is reset through an OR circuit 243.

In addition, an NMOS 246 is turned off at a point in time when thedetection current IS exceeds a reference voltage Vref4 of a comparator213 at the time of starting. Consequently, the resistor 135 isdisconnected and frequencies of the first and second triangular wavesignals CT, CTI for PWM are changed.

An REG block 207 generates an adjusting voltage VREG of a more stablepredetermined voltage level from the power source voltage VCC, andsupplies the adjusting voltage to each of the necessary circuits.

The detection current IS proportional to a current flowing through thecold cathode fluorescent lamp FL is inputted to a first error amplifier211 and is compared with a reference voltage Vref2 (for example, 1.25 V)and an NPN 235 connected to a constant-current source I1 is controlledby an output according to its error. A collector of this NPN 235 isconnected to the eighth pin 8P and a potential of this connection pointbecomes the feedback voltage FB and is inputted to the PWM block 250 asa comparison input.

The first triangular wave signal CT, the second triangular wave signalCTI, the feedback voltage FB and the slow start voltage SS are inputtedin the PWM block 250.

FIG. 3 is a diagram showing a circuit configuration example of the PWMblock 250. In FIG. 3, the PWM block 250 has a first PWM comparator 251and a second PWM comparator 252.

The first triangular wave signal CT, the feedback voltage FB and theslow start voltage SS are inputted to the first PWM comparator 251, andthe first triangular wave signal CT is compared with a lower voltageamong the feedback voltage FB and the slow start voltage SS, and a firstcomparison result output (a first pulse width modulation signal) n1 isoutputted.

The second triangular wave signal CTI, the feedback voltage FB and theslow start voltage SS are inputted to the second PWM comparator 252, andthe second triangular wave signal CTI is compared with a lower voltageamong the feedback voltage FB and the slow start voltage SS, and asecond comparison result output (a second pulse width modulation signal)n2 is outputted.

Description will be made again by returning to FIG. 2. The firstcomparison result output n1 from the PWM block 250 and a signal from thereverse circuit 249 are inputted to the logic block 203 through an ANDcircuit 247 with a reverse input terminal. Also, the second comparisonresult output n2 from the PWM block 250 and a signal from the reversecircuit 249 are inputted to the logic block 203 through an AND circuit248 with a reverse input terminal.

The first and second switch driving signals N1, N2 are formed by thelogic block 203 and the output block 204 based on these first and secondcomparison result outputs n1, n2 or the clock CLK.

Also, the capacitor 136 is connected between the eighth pin 8P and theninth pin 9P, so that the feedback voltage FB increases or decreasessmoothly. Therefore, the PWM control is performed smoothly withoutshock.

The detection voltage VS proportional to a voltage applied to the coldcathode fluorescent lamp FL is inputted to a second error amplifier 212and is compared with a reference voltage Vref3 (for example, 1.25 V) andan NPN 238 with a double collector structure in which one doublecollector is connected to the constant-current source I1 is controlledby an output according to its error. A collector of this NPN 238 is alsoconnected to the eighth pin 8P, so that the feedback voltage FB is alsocontrolled by the detection voltage VS. In addition, when the feedbackvoltage FB exceeds a reference voltage Vref1 (for example, 2.4 V), a PNPtransistor (hereinafter called a PNP) 231 is turned on and an excessiveincrease in the feedback voltage FB is limited.

A comparator 215 compares a reference voltage Vref7 with a voltage inwhich the power source voltage VCC is divided by resistors 240, 241 andat a point in time when the power source voltage VCC reaches apredetermined value, its output is reversed and the FF circuit 242 isreset through the OR circuit 243. Also, an output of the comparator 215is supplied to the logic block 203.

A comparator 218 compares the slow start voltage SS with a referencevoltage Vref8 (for example, 2.2 V) and when the slow start voltage SSincreases, the NPN 234 is turned on through an AND circuit 244 and theOR circuit 239. By turning on the NPN 234, a diode 232 is backwardbiased by a current source I2 and as a result of that, a normaloperation of the first error amplifier 211 can be performed. Inaddition, a series circuit of a diode 237 and a PNP 236 is connectedbetween a ground and a collector of the NPN 234 and the referencevoltage Vref2 is applied to a base of the PNP 236.

When the NPN 238 in which the other double collector is connected to aconstant-current source I3 is turned on by the second error amplifier212, a voltage of a comparator 219 becomes lower than a referencevoltage Vref9 (for example, 3.0 V) and a comparison output is reversed.A comparator 220 compares the feedback voltage FB with a referencevoltage Vref10 (for example, 3.0 V) and when the feedback voltage FBbecomes higher, a comparison output is reversed.

Outputs of the comparators 219, 220 and an output of the comparator 213are inputted to the AND circuit 244 through an OR circuit 245. An outputof the AND circuit 244 is applied to a timer block 206 and apredetermined time is measured to produce an output. The FF 242 is setby an output of this timer block 206 and an operation of the logic block203 is stopped by a Q output of this FF circuit 242. Also, the Q outputof the FF circuit 242 is supplied to the nineteenth pin P19 as a FAILsignal through a reverse circuit 222.

Next, an operation of the inverter constructed as mentioned above willbe described with reference to a timing chart of FIG. 4.

While the duty signal DUTY exceeds the triangular wave signal BCT forburst, PWM control is performed. On the other hand, while the dutysignal DUTY falls below the triangular wave signal BCT for burst, thePWM control is stopped and electric power supply to the cold cathodefluorescent lamp FL is stopped.

A frequency of the triangular wave signal CT for PWM is, for example, 50kHz and burst control of this signal CT is performed by the triangularwave signal BCT with a frequency of, for example, 150 Hz, so that thereis no problem in vision. Then, by controlling the magnitude of the dutysignal DUTY, electric power supply, that is, light quantity can becontrolled more widely beyond the range capable of being supplied to thecold cathode fluorescent lamp FL by only the PWM control.

Referring concretely to a circuit operation, while the duty signal DUTYfalls below the triangular wave signal BCT for burst, an output of thecomparator 221 is at a low (L) level and the NPN 234 is in an off state.

Consequently, the diode 232 is forward biased by the constant-currentsource I2 and an input of the first error amplifier 211 becomes a highvalue and the NPN transistor 235 is turned on and the feedback voltageFB is regulated to a low voltage.

The first PWM comparator 251 compares a positive (+) first triangularwave signal CT with a lower voltage among two negative (−) inputs. Also,the second PWM comparator 252 compares a positive (+) second triangularwave signal CTI with a lower voltage among two negative (−) inputs.Therefore, in this case, the first and second comparison result outputs(first and second pulse width modulation signals) n1, n2 are notoutputted.

Next, when the duty signal DUTY exceeds the triangular wave signal BCTfor burst, the NPN 234 is turned on and the diode 232 is backward biasedand is turned off. At this time, the detection current IS is at a lowvalue, so that the first error amplifier 211 generates an outputaccording to the inputted detection current IS and continuity of the NPN235 is controlled.

A collector voltage of its NPN 235, that is, the feedback voltage FBgradually increases by action of the capacitor 136 connected between theeighth pin and the ninth pin and reaches a steady-state value accordingto the original feedback. Consequently, the first and second pulse widthmodulation signals n1, n2 are supplied from the PWM block 250 to thelogic block 203 and the gate driving signals N1, N2 are outputted andPWM control of the NMOSs 101, 102 is performed.

Next, an operation and a configuration of a parallel running system ofthe inverters constructed as mentioned above will be described withreference to FIG. 5. FIG. 5 is a circuit diagram for explanation inwhich the portion related to an operation of a parallel running systemof the inverters is taken out of FIGS. 1 and 2 and a mutual connectionrelation between each of the inverters is shown.

Plural inverters run in parallel are respectively disposed close toplural CCFLs arranged in each place of a liquid crystal display. Ofcourse, one inverter may be associated with two or more CCFLs. In thiscase, plural secondary windings of the transformer TR of FIG. 1 are usedand power is supplied from the respective secondary windings to the coldcathode fluorescent lamp FL. Or, it may be constructed so that PWMcontrol circuit parts of plural systems are disposed in the controllerIC 200 of FIG. 1 and PWM driving signals of plural systems areoutputted.

In FIG. 5, all the controller ICs 200A to 200N of each of the invertershave the same internal configuration, so that the controller IC 200Awill be described typically.

In an OSC block 201, a first signal line to output a first triangularwave signal CT for PWM is connected to an external terminal 4P and also,a second signal line is connected to an external terminal 5P. Also, theOSC block 201 outputs a second triangular wave signal CTI for PWM and aclock signal CLK with the same frequency in synchronization with thefirst triangular wave signal CT. The first and second triangular wavesignals CT, CTI are supplied to a PWM block 250 and the clock signal CLKis supplied to a logic block 203.

A comparator 217 compares a potential STB of an external terminal 11Pwith a reference voltage Vref6 and while the potential STB exceeds thereference voltage Vref6, a system of the controller IC 200A is turnedon. In reverse, while the potential STB is lower than the referencevoltage Vref6, the system of the controller IC 200A is turned off.

In the controller ICs 200A to 200N constructed thus, the externalterminals 4P are mutually connected to the external terminals 11P,respectively.

When the controller IC 200A is used as a main controller, a capacitor133 for frequency decision is connected to the external terminal 4P ofthe controller IC 200A between a ground and the external terminal 4P,and a resistor 134 for frequency decision is connected to the externalterminal 5P between the ground and the external terminal 5P, and astarting resistor 135 is connected between the external terminal 5P andan external terminal 6P. Further, a starting signal STB is supplied tothe external terminal 11P. These resistors and capacitor are notconnected to the controller ICs 200B to 200N used as sub-controller ICs.

In the parallel running of the inverters constructed as described above,when the starting signal STB is set at a H level, an output of thecomparator 217 reverses from the H level to a L level and a system ofeach of the controller ICs 200A to 200N is turned on.

By turning on the system, the OSC block 201 of the main controller IC200A generates the clock signal CLK and the first and second triangularwave signals CT, CTI with a relatively high frequency for startingdecided by the capacitor 133 for frequency decision, the resistor 134for frequency decision and the starting resistor 135.

The system of the sub-controllers 200B to 200N is turned on almostsimultaneously with the main controller IC 200A, but the resistor 134for frequency decision (of course, the starting resistor 135) is notconnected to the external terminal 5P, so that the first and secondtriangular wave signals CT, CTI and the clock signal CLK are notgenerated themselves.

The first triangular wave signal CT generated in the main controller200A is supplied to the sub-controller ICs 200B to 200N connectedmutually respectively. In the sub-controller ICs 200B to 200N, thesecond triangular wave signal CTI and the clock signal CLK are formedbased on the first triangular wave signal CT supplied from the maincontroller IC 200A.

Consequently, sub-inverters operate in synchronization with a maininverter having the main controller 200A, so that all the invertersoperate synchronously in the same phase. Then, the respective inverterscan be arranged close to CCFLs (backlight light sources of a liquidcrystal display) distributed and arranged.

Therefore, the plural CCFLs synchronously emit light, so that asituation in which the light mutually interferes and flickers can beprevented. Also, each of the inverters is arranged close to each of theCCFLs, so that an influence on other apparatus by routing high-voltagewiring can be reduced. Also, parasitic capacitance of the CCFL can beeffectively used in resonance with a transformer.

Particularly, only the first triangular wave signal CT is simplytransmitted between plural inverters to run in parallel for synchronousrunning, so that the number of connection lines between each of themutual inverters can be reduced.

In addition, the above embodiment is constructed so that the externalterminals 11P of all the controller ICs 200A to 200N are mutuallyconnected and all the controller ICs, that is, all the inverters arestarted and stopped in common. Instead of this, it may be constructed sothat a starting signal ST is given to individual controller ICs or agrouped controller IC group and inverters are started and stoppedseparately. Also in this case, synchronous running of the inverters canbe performed without trouble.

Thus, each of the inverters is arranged close to plural loads requiringa high voltage and also an operation or a non-operation of oscillationis decided by the fact as to whether or not a resistor for frequencydecision is connected to each of the inverters. Then, by disposing theOSC block 201 for forming the clock signal CLK and the second triangularwave signal CTI based on the first triangular wave signal CT suppliedfrom the outside, the plural loads can easily be controlledsynchronously in the same phase using one inverter as a master and theother inverters as a slave.

Also, the same controller IC is used in each of the inverters andpredetermined terminals with the same function are mutually connected incommon and only a main controller IC is made to perform an oscillationoperation, so that a configuration of the whole system is simplified andalso the number of loads used is not limited.

Next, an operation of the inverters constructed as mentioned above willbe described with reference to an operating state diagram at each timingof FIG. 6 and waveform diagrams of FIGS. 7 and 8 together with thetiming chart of FIG. 4.

As shown in FIG. 4, at a point in time of one vertex (for example, thelower vertex) of the first triangular wave signal CT, the first switchdriving signal N1 is turned on and is at a H level until the firsttriangular wave signal CT immediately after the point in time becomesequal to the feedback signal FB. Since the first semiconductor switch101 is turned on by the H level of the first switch driving signal N1,the on state is continued for only this period.

Also, at a point in time of one vertex (for example, the lower vertex)of the second triangular wave signal CTI, the second switch drivingsignal N2 is turned on and is at a H level until the second triangularwave signal CTI immediately after the vertex becomes equal to thefeedback signal FB. Since the second semiconductor switch 102 is turnedon by the H level of the second switch driving signal N2, the on stateis continued for only this period.

Then, the first and second switch driving signals N1, N2 are generatedat timing at which an off period during which all the first and secondsemiconductor switches 101, 102 are turned off is set between periodsfor which the first semiconductor switch 101 and the secondsemiconductor switch 102 are alternately turned on and the switches arealternately in the on states

At an interval i of FIG. 4, the gate driving signal N1 is at a H leveland the NMOS 101 is turned on and a current flows through the primarywinding 107 from the DC power source BAT (power source voltage VDD) in afirst direction from the center tap T through the first terminal A. Thisstate is shown in FIG. 6( i). A voltage (hereinafter called an A pointvoltage) Va of the first terminal A is the ground voltage GND as shownin FIG. 7, and a current does not flow through a first snubber circuitmade of a series circuit of the resistor 103 and the capacitor 104.

In the case of reaching an interval ii, the gate driving signal N1becomes a L level and all the NMOSs 101, 102 become an off state. Thisstate is shown in FIG. 6( ii). At this interval ii, a current of thefirst direction flows through the DC power source BAT (power sourcevoltage VDD) through the second terminal B and the center tap T from asecond snubber circuit made of a series circuit of the resistor 105 andthe capacitor 106 or a body diode of the NMOS 102 by the stored energyof the transformer TR. In this state, the A point voltage Va becomes avoltage (2×VDD+Vf) in which a voltage Vf dropped by the body diode isadded to two times the power source voltage VDD as shown in FIG. 7.

At an interval iii, the gate driving signal N2 is at a H level and theNMOS 102 is turned on and a current flows through the primary winding107 from the DC power source BAT (power source voltage VDD) in a seconddirection from the center tap T through the second terminal B. Thisstate is shown in FIG. 6( iii). The A point voltage Va is two times(2×VDD) the power source voltage VDD as shown in FIG. 7.

In the case of reaching an interval iv, the gate driving signal N2becomes a L level and all the NMOSs 101, 102 become an off state. Thisstate is shown in FIG. 6( iv). At this interval iv, a current of thesecond direction flows through the DC power source BAT (power sourcevoltage VDD) through the first terminal A and the center tap T from afirst snubber circuit or a body diode of the NMOS 101 by the storedenergy of the transformer TR. In this state, the A point voltage Vabecomes a voltage (−Vf) lower than the ground voltage by a voltage Vfdropped by the body diode as shown in FIG. 7.

FIG. 7 is a diagram showing a load current Io and an A point voltage Vain the embodiment of the invention provided with first and secondsnubber circuits, and FIG. 8 shows a load current Io and an A pointvoltage Va in correspondence with FIG. 7 in the case where first andsecond snubber circuits are not disposed.

In FIG. 7 showing characteristics of the invention, an upper limit ofthe A point voltage Va becomes a voltage (2×VDD+Vf) in which a voltageVf dropped by a body diode is added to a voltage two times the powersource voltage VDD. On the other hand, in FIG. 8 showing characteristicsof the case where the first and second snubber circuits are notdisposed, in the A point voltage Va, a very high abnormal voltage Vpeakwith spike shape occurs at the time of entering an interval ii. As aresult of measurement, this abnormal voltage Vpeak becomes about 5 to 6times the power source voltage VDD.

When this abnormal voltage Vpeak occurs, it is necessary to use anelement designed to withstand a high voltage capable of withstanding theabnormal voltage Vpeak and also, the abnormal voltage Vpeak results in anoise generation source to the periphery.

Particularly, in a power source circuit with a push-pull configurationas described in the invention, a voltage two times the power sourcevoltage VDD is always applied to the terminals A, B, that is, the switchelements 101, 102, so that suppression of the abnormal voltage reaching5 to 6 times the power source voltage VDD is particularly effective.

FIG. 9 is a diagram showing another configuration example of a snubbercircuit. In FIG. 9, a first snubber circuit made of a series circuit ofa resistor 103 and a capacitor 104 is disposed between a center tap Tand one end A of a primary winding 107 and also, a second snubbercircuit made of a series circuit of a resistor 105 and a capacitor 106is disposed between the center tap T and the other end B of the primarywinding 107.

Also in an inverter of this FIG. 9, an abnormal overvoltage occurring interminals A, B of the primary winding 107 can be suppressed in a mannersimilar to the first and second snubber circuits in FIG. 1. That is, thefirst and second snubber circuits could be connected between each of theends A, B of the primary winding 107 and a potential point VDD, GND withlow impedance. In addition, the resistor 103, 105 for snubber may beomitted in each of the snubber circuits of FIG. 1 or FIG. 9.

1. A controller IC for controlling a semiconductor switch for driving aload, the controller comprising: an oscillator block for generating afirst triangular wave signal and a second triangular wave signal, saidsecond triangular wave signal being reversed relative to the firsttriangular wave signal; a current detection signal input terminal towhich a current detection signal is inputted, said current detectionsignal being based on a current flowing through the load; a PWM controlsignal generation circuit for generating a PWM control signal forswitching the semiconductor switch based on the first triangular wavesignal, the second triangular wave signal, and an error signal, saiderror signal being generated based on the current detection signal; aburst signal generation circuit for generating a burst signal in which afirst state and a second state are repeated; and a burst control circuitcoupled to the burst signal generation circuit inside the controller IC,wherein the burst control circuit halts a switching operation of thesemiconductor switch when the burst signal is in the first state.
 2. Thecontroller IC of claim 1, wherein the oscillator block includes acapacitor connecting terminal to which a capacitor for frequencydecision is connected.
 3. The controller IC of claim 1, wherein theoscillator block outputs triangular wave signals from the capacitorconnecting terminal when the capacitor for frequency decision is notconnected.
 4. The controller IC of claim 1, wherein the burst signal isinputted to the PWM control signal generation circuit.
 5. The controllerIC of claim 1, wherein the burst signal generation circuit is coupled tothe current detection signal input terminal, and wherein the burstsignal generation circuit includes: a current source for supplying aconstant current; a switch coupled to one end of the current source andperforming ON/OFF operation based on the burst signal; and a diode usedto supply the constant current of the current source to the currentdetection signal input terminal when the burst signal is in the firststate.
 6. The controller IC of claim 1, wherein the PWM control signalgeneration circuit includes an error amplifier for generating the errorsignal based on the current detection signal and a reference signal, andwherein the PWM control signal generation circuit switches thesemiconductor switch based on the error signal.
 7. The controller IC ofclaim 1, wherein the error signal is set to almost zero by the burstsignal when the burst signal inputted to the error amplifier is in thefirst state.
 8. The controller IC of claim 1, further comprising: anerror signal output terminal for outputting the error signal to theoutside of the controller IC.
 9. The controller IC of claim 1, wherein acapacitor is coupled between the error signal output terminal and thecurrent detection signal input terminal.
 10. An AC-DC convertercomprising: a transformer having a primary winding with a center tap andat least one secondary winding, the center tap being coupled to a firstpotential point of a DC power source; a first semiconductor switchcoupled between one end of the primary winding and a second potentialpoint of the DC power source; a second semiconductor switch coupledbetween the other end of the primary winding and the second potentialpoint; a current detection circuit for detecting a current flowingthrough a load coupled to the secondary winding; an oscillator block forgenerating a first triangular wave signal and a second triangular wavesignal, said second triangular wave signal being reversed relative tothe first triangular wave signal; a current detection signal inputterminal to which a current detection signal is inputted, said currentdetection signal being based on a current flowing through the load; aPWM control signal generation circuit for generating a PWM controlsignal for switching the semiconductor switch based on the firsttriangular wave signal, the second triangular wave signal, and an errorsignal, said error signal being generated based on the current detectionsignal; a burst signal generation circuit for generating a burst signalin which a first state and a second state are repeated; and a burstcontrol circuit coupled to the burst signal generation circuit insidethe controller IC, wherein the burst control circuit halts a switchingoperation of the semiconductor switch when the burst signal is in thefirst state.